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I've been running DD-WRT at home for well over three years on a couple different Linksys WRT54G's. The features I like:

OpenVPN server or client. When I was with a previous company, I had it set up to connect to the work OpenVPN. I've also set it up as a server so I can access anything on my home LAN.

Bridging the wifi to the LAN, aka Wireless Bridge. Clients on the wireless are on the same subnet as the rest of the LAN, meaning no crazy DNS or NAT tricks to talk seamlessly between systems. This means you can do cool things like multi-MAC IP assignments, so clients that are sometimes wireless, sometimes wired can have the same IP (with an external DHCP server).

Administration interface is easier to navigate than the stock Linksys firmware.

Port forwarding knows no bounds. The default FW on both my Linksys WAPs could only do something like 10 or 12 port forwards. I need more than that for gaming and for work purposes.

Status pages with data pr0n. The sys-info default page is nice with useful information. The real-time graphs on the bandwidth page are excellent.

It feels (and is :-)) more feature-complete than the stock firmware, at least on Linksys devices.

You can pass additional commands to the system shell directly, further customizing beyond even DD-WRT's extensive capabilities.

You can get a version of Tomato that has a more recent version of pppd, allowing MLPPP so you can bind several DSL lines. Or you can use it over just one line to evade Bell Nexxia's traffic shaping.

The biggest reason I flashed my router was because the default limit of 512 open sockets on the WRT54G is too little. If you have a couple torrents going your router will easily lock up after 24 hours. Upping that to 4096 seems to have fixed things.

I also got a lot of use out of the wireless bridge feature to connect my entertainment centre to wifi for $40, rather than $80 per device.